When Tickled, Rats Giggle at a Supersonic Level

The foundations of true happiness for mammals, including humans, are being revealed by a surprising group: giggling, ticklish rats.

When rats are in a good mood, they enjoy tickling so much that they emit ultrasonic giggles, according to a new study in Science that identifies the neurons in the brain tied to ticklishness and laughter. Stimulating these neurons, located in the somatosensory cortex - near the center of the brain - causes rats to laugh. The finding suggests that future therapies for people suffering from depression might target this brain region.

It's hard to imagine animals as low on the food chain as rats having much in common with happy humans, but co-author Michael Brecht of Humboldt University of Berlin told Seeker that "rat tickling and tickling-evoked positive emotions in rats should be viewed as primitive forms of joy."

Animals may even have a sense of humor. The notion has been challenging to assess. Rats, after all, aren't yukking it up over our jokes. Brecht said no one has been able to properly investigate humor in animals, yet "there are claims that chimpanzees can gesture in a humorous way."

Tickling has turned out to be a key way to investigate pleasure shared by mammals since many seem to like to this form of touch, but only some of the time. As famed naturalist Charles Darwin once wrote: "the mind must be in a pleasurable condition" for ticklish laughter to occur.

Brecht and co-author Shimpei Ishiyama found this to be the case for rats. When the rodents were stressed out, such as when they were placed on an uncomfortably high platform, their tickling-evoked laughter and corresponding neuron firing were significantly suppressed.

Humans are no different. If a situation or person makes you uncomfortable, you will clearly not welcome their tickling you.

The new findings "suggest a link between tickling and play," Brecht said. He now thinks that "ticklishness is a trick of the brain to make animals or humans interact in a fun way." Rats also appear to laugh during play that doesn't even involve touching.

It has been reported that chimpanzees laugh when they experience welcome tickles. "Also," Brecht said, "ticklishness in dogs is commonly seen and appears to be joyful and linked to play."

Even meerkats laugh when they are tickled:

"This is not to say that all mammals show ticklishness," he said. "Somewhat to our surprise, we were unable to demonstrate ticklishness in mice."

He suspects that suppression of fun-related behaviors in mammals could be adaptive, meaning that we protect ourselves against unwanted touch, interactions and more when we are in a state of unease. It could be that our brains get stuck in this more miserable state, either through a lot of prior stress or perhaps due to inherited tendencies.

What's more evident is that humans aren't the only animals who may literally jump for joy. Rats appear to leap in joy too, as do sheep, guinea pigs and rabbits. As a result, Brecht thinks that, much like responses to ticklishness, jumping for joy represents "a very old mammalian behavior."

David Kleinfeld of the UC San Diego Physics Department and his lab team also study what goes on at the neuronal level in the brains of animals as they engage in various behaviors.

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He believes that the new study on rats "is a wonderful and pioneering paper" that helps explain what underlies "play and the vocalization that accompanies play."

Image: The somatosensory cortex highlighted in an illustration of the brain, showing where neurons tied to ticklishness and laughter are located. Credit: Shimpei Ishiyama and Michael Brecht

Kleinfeld told Seeker that he wasn't surprised that a state of anxiety versus serenity modulates the response to tickling.

On the other hand, "An unexpected finding is that play, in the absence of touch, leads to calls of pleasure," Kleinfeld said. "This suggests a kind of mental imagery of tickle: anticipation is enough to activate neurons presumably by changes in the threshold of neuronal firing by dopamine and other neuromodulators."

Dopamine is often referred to as being one of the "happy hormones" that is responsible, at least in part, for our experiencing happiness.